Tab completion is incredibly slow and picky about formatting

Describe the bug

Tab completion is painfully slow. See repro.

Command Name az

Errors:

To Reproduce:

1. Properly configure tab completion for your shell, I’m in zsh but this all holds true in bash as well.
2. Type az in the terminal
3. Hit tab, it adds a space instead of giving you tab completion.
4. Hit tab again, wait 1-2 seconds for tab completion to process
5. After a wait, tab completion displays a list of completions.

Another example:

1. Type az devops or az boards in the terminal
2. Hit tab, wait ~5 seconds
3. Get -- no matches found -- output.
4. Hit space, hit tab
5. Wait ~1s get a bunch of completions

Expected Behavior

A response should be given immediately with no delay and it should not be picky about whether there is a space after the last command.

The entire autocompletion process is way too slow. Type git and hit tab and you get an instant response with completions with nice formatting. This has got to be better in the azure cli. 1-2s is too long, but 5 seconds is just shameful especially having it return no results after, only to work after hitting space.

Please let me know if you need any more information to help prioritize this work, feel free to reach out to me via email or teams via my alias: jiboulte.

Environment Summary

Darwin-18.2.0-x86_64-i386-64bit
Python 3.7.3
Shell: bash

azure-cli                         2.0.62

Extensions:
azure-devops                       0.4.0

Python location '/usr/local/Cellar/azure-cli/2.0.62/libexec/bin/python'
Extensions directory '/Users/jiboulte/.azure/cliextensions'

Python (Darwin) 3.7.3 (default, Apr 19 2019, 09:07:31) 
[Clang 10.0.1 (clang-1001.0.46.3)]

Additional Context

[nadi-noda]

13/02/2021 still too slow

[jiasli]

The slowness is due to the fact that auto-complete is achieved by calling az again with the prefix given in the terminal. Azure CLI now contains thousands of commands and loading these commands makes it slow.

We will try to improve the performance in the future.